Charge coupled devices (CCDs) have been utilized as solid state shift registers for a number of years. See, for example the article appearing in the June 21, 1971, issue of ELECTRONICS MAGAZINE on pages 50-59.
For CCDs, as well as other types of recirculating memories, such as delay lines, information is generally read out sequentially at a particular tap point, or else an entire word of data is read out in parallel from a plurality of adjacent tap points. In either case, the user has to wait for the data to reach the fixed tap point or tap points, and circuitry must be provided to keep track of the time at which the addressed information is available at the tap point or tapped points. Consequently, it has generally been impractical to implement random access memories using serial memory devices, especially CCDs.